Numerous processes are known in the art for the production of a nitrogen product stream by using cryogenic distillation. The conventional process for the production of pressurized nitrogen directly from a cryogenic separation zone uses a single pressure distillation column with the oxygen rich waste stream being used at least in part to provide the process refrigeration by work expansion. Details of such processes are disclosed in U.S. Pat. No. 4,222,756.
U.S. Pat. No. 4,848,996 discloses an improvement to a standard nitrogen generator. The improvement is two-fold; first, the addition of one or more distillation stages above the reboiler, which stages effectively transform the reboiler/condenser into a partial low pressure column and allow further separation (rectification) of the nitrogen generator bottoms liquid into two streams. Second, the recycle of the overhead stream (at a composition close to that of air) from the top of the low pressure column to the main air compressor. Additionally, at least a portion of the oxygen-enriched stream that exits the low pressure column below the bottom tray is expanded to provide refrigeration for the cycle.
U.S. Pat. No. 4,872,893 discloses a process for the recovery of nitrogen from a feed gas stream, containing nitrogen and oxygen, using a cryogenic separtion wherein a recycle stream having an oxygen content above that of the feed gas is recycled from the cryogenic separation to the feed gas stream without any intervening process step that would decrease the oxygen content of the recycle stream.
U.S. Pat. No. 4,872,893 discloses a process for the recovery of nitrogen from a feed gas stream, containing nitrogen and oxygen, using a cryogenic distillation wherein a recycle stream having an oxygen content above, equal to or below that of the feed gas stream is recycled from the cryogenic separation to the feed gas stream with a split reboiler/condenser function that would allow variation of the oxygen content of the recycle stream.